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CS 635: Advanced Object-Oriented Design & Programming |
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Spring Semester, 1998
Observer Variants
To Lecture Notes Index
© 1998, All Rights Reserved, SDSU & Roger Whitney
San Diego State University -- This page last updated 21-Apr-98
Contents of Doc 8, Observer Variants
| References | slide # 1 |
| |
| Conduit of Subject Change | slide # 2 |
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| Event Channel - OMG Event Service
Specification | slide # 4 |
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| Reactor Pattern | slide # 6 |
References
Conduit of Subject Change Pattern
- "Implementation Patterns for the Observer Pattern", Kim and
Benner, in Pattern Languages of Program Design 2, Edited by
Vlissides, Coplien, Kerth, 1996, pp 75-86
Event Channel
- Pattern-Oriented Software Architecture: A System of Patterns
,
Buschmann, Meunier, Rohnert, Sommerlad, Stal, Wiley,1996, pp 341
343
Inside Corba: Distributed Object Standards and Applications,
Mowbray & Ruh, Addison-Wesley 1997, pp 112-116
Reactor
- Reactor: An Object Behavioral Pattern for Concurrent Event
Demultiplexing and Event Handler Dispatching, Schmidt, in Pattern
Languages of Program Design 1, Edited by Coplien, Schmidt, 1995, pp
529-545
Observer Variants and Similar Patterns
Conduit of Subject Change
Problem: In complex subjects it can be difficult for the
observer to extract detailed change information from the
subject. In such situations the observer becomes tightly
coupled with the subject.
Goal: Provide a uniform mechanism for sending state
change information on a subject to the observer via a
message
Solution: the subject sends a structured update
message object to the observer
The update message contains four lists
- add list
- remove list
- modified list
- container list
Add List contains all objects added to the state in the
change
Remove List contains all objects remove from the state
in the change
Modified List contains all objects modified in the state
in the change
Container List contains the containers that hold the
objects that were added, removed, or modified
Example
Let the subject be a car object that contains a linked list
of tire objects, label it tires
Assume that the car just had two flat tires, label them
frontLeft and rearLeft
The car would create a new update message object,
- the add list and the remove list would be empty
- the modified list would contain frontLeft and
rearLeft
- the container list would contain tires
The observer receiving the update message object would
then search the lists to determine which parts of the car
object changed and act appropriately
Event Channel - OMG Event Service Specification
Subject (Supplier, Publisher) changes state (or produces
an item)
Observer (Consumer, Subscriber) reacts to the change in
state (or consume the item)
Classic Supplier Consumer
Push verses Pull Model
Push
- When supplier (publisher) creates a new item, it
pushes it to the consumer (subscriber) whether they
want it or not
Pull
- When a consumer (subscriber) needs an item it
requests it from the supplier (publisher), whether
the item is ready or not
Event Channel
When a push supplier pushes an item the event channel
pushes the item to all its push consumers and saves the
item until all its pull consumers have pulled the item
The event channel must periodically pull from all its pull
suppliers. The pull can be initiated by a pull consumer or
the event channel.
Reactor Pattern
This pattern supports in a single process the
demultiplexing and dispatching of multiple event
handlers which are triggered concurrently by multiple
events. The reactor pattern simplifies event-driven
applications by integrating the demultiplexing of events
and the dispatching of the corresponding even handlers.
This pattern is of interest to those who need to
implement a concurrent server in an environment that
does not support threads or multiple processes.
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